1. Field of the Invention
The present invention relates to a wideband wireless communication system using an Orthogonal Frequency Division Multiplexing (OFDM) scheme, and more particularly, to an apparatus and method for transmitting/receiving a pilot code pattern to identify a base station.
2. Background of the Prior Art
In an Orthogonal Frequency Division Multiplexing (OFDM) communication system, a transmitter (i.e. a base station (BS)) transmits a pilot sub-carrier (hereinafter, referred to as “pilot channel”) signals to a receiver (i.e. a mobile station (MS)). The base station transmits data sub-carrier (hereinafter, referred to as “data channel”) signals together with the pilot channel signals. The pilot channel signals are transmitted for synchronization acquisition, channel estimation, and the identification of a base station.
In an OFDM scheme used for high-speed data transmission over wired/wireless channels, data is transmitted using multi-carriers. The OFDM scheme is a kind of a Multi-Carrier Modulation (MCM) scheme for parallel-converting a serial input symbol sequence and modulating the parallel-converted symbols to mutually orthogonal sub-carriers, that is, mutually orthogonal sub-channels.
The MCM system was applied to a military high frequency (HF) radio communication in the late 1950's. The OFDM scheme with overlapping orthogonal sub-carriers was initially developed in the 1970's, but it was difficult to implement the orthogonal modulation between multi-carriers. Therefore, the OFDM scheme had a limitation in the real system implementation.
In 1971, Weinstein et al. proposed that OFDM modulation/demodulation can be efficiently performed using a Discrete Fourier Transform (DFT), which was a driving force behind the development of the OFDM scheme. Also, the introduction of a guard interval and a cyclic prefix as a specific guard interval further mitigated adverse effects of the multi-path propagation and the delay spread on the systems.
Accordingly, the OFDM scheme has been exploited in various fields of digital data communications such as Digital Audio Broadcasting (DAB), digital TV broadcasting, Wireless Local Area Network (WLAN), and Wireless Asynchronous Transfer Mode (WATM). Although hardware complexity was an obstacle to the wide use of the OFDM scheme, recent advances in digital signal processing technology including a Fast Fourier Transform (FFT) and an Inverse Fast Fourier Transform (IFFT) simplified the implementation of the OFDM scheme.
OFDM, similar to a Frequency Division Multiplexing (FDM) scheme, boasts of an optimum transmission efficiency in a high-speed data transmission because it transmits data on the sub-carriers, while maintaining orthogonality between them. The overlapping use of the frequency spectrums leads to an efficient frequency use, and a robustness against the multi-path fading and the frequency selective fading further increase the transmission efficiency in the high-speed data transmission.
Also, the OFDM scheme can reduce the effects of the Inter Symbol interface (ISI) by using guard intervals, which enables the design of a simple equalizer hardware structure. Furthermore, since OFDM is robust against impulsive noise, it is increasingly utilized in communication system configuration.
The pilot channel signals serve as a training sequence and are used for channel estimation between the transmitter and the receiver. Further, the mobile stations can identify their base station by using the pilot channel signals. The locations where the pilot channel signals are transmitted are previously agreed between the transmitter and the receiver. Consequently, the pilot channel signals serve as a reference signal.
A transmission pattern of the pilot channel signal transmitted from the base station is referred to as the pilot pattern. In the OFDM communication system, the pilot pattern is identified by a slope of the pilot channel signals and a transmission start point of the pilot channel signals. The OFDM communication system is designed such that the base stations have their own unique pilot patterns for base station identification.
In addition, the pilot pattern is generated by considering a coherence bandwidth and a coherence time. The coherence bandwidth represents a maximum bandwidth where a channel is constant in a frequency domain, and the coherence time is a maximum time where a channel is constant in a time domain.
Because it can be assumed that the channel is constant within the coherence bandwidth and the coherence time, even though only one pilot channel signal is transmitted within the coherence bandwidth and the coherence time, it is sufficient for synchronization acquisition, channel estimation, and the identification of the base station. Consequently, it is possible to maximize the transmission of the data channel signals, thereby contributing to the improvement in the entire system performance. A minimum frequency interval for transmitting the pilot channel signals is the coherence bandwidth, and a minimum time interval, or a minimum OFDM symbol time interval, for transmitting the pilot channel signals is the coherence time.
The number of the base stations constituting the OFDM communication system is varied depending on size of the OFDM communication system. The number of the base stations increases as the size of the OFDM communication system increases. Therefore, in order to identify all of the base stations, the number of the pilot patterns having different slopes and different start points should be equal to the number of the base stations.
FIG. 1 is a diagram illustrating the possible pilot patterns that can be generated within a coherence bandwidth and a coherence time in a conventional OFDM communication system.
Referring to FIG. 1, the number of possible pilot patterns (that is, the number of possible slopes for the transmission of the pilot channel signal) is limited according to a coherence bandwidth 201 and a coherence time 202. Assuming that a coherence bandwidth 201 is 6, a coherence time 202 is 1 and a slope of a pilot pattern is an integer, there are six possible slopes of s=0 to s=5 for a pilot pattern. A slope for a pilot pattern becomes one of integers 0 to 5. Because the number of possible slopes for a pilot pattern is six, the number of base stations that can be identified using the pilot pattern in the OFDM communication system is six.
In the case where the slope of the pilot pattern is s=6, the slope of s=6 is not distinguished from the slope of s=0, so that only one of the two slopes of s=0 and s=6 is used. In other words, since a slope of s=6 for a pilot pattern has the same pattern as the slope of s=0 for another pilot pattern separated by a coherence bandwidth, the slopes of s=0 and s=6 cannot be distinguished from each other. Therefore, the slopes of the pilot sub-carriers are limited to the coherence bandwidth. In FIG. 1, a shaded circle represents a pilot channel signal separated by the coherence bandwidth 201.
An Orthogonal Frequency Division Multiple Access—Code Division Multiplexing (hereinafter, referred to as “ODFMA-CDM”) system will now be described briefly.
FIG. 2 is a diagram illustrating a method for allocating time-frequency resources in an OFDMA-CDM system.
Referring to FIG. 2, a unit rectangle is a time-frequency cell (TFC) having a frequency bandwidth ΔfTFC corresponding to a predetermined number of sub-carriers (e.g., eight sub-carriers) and having a time duration ΔtTFC corresponding to a predetermined number of OFDM symbol interval (e.g., one OFDM symbol interval). The number of the sub-carriers constituting the TFC can be variably set depending on system environment. The frequency bandwidth and the time duration occupied by the TFC will be referred to as “TFC frequency interval” and “TFC time interval”, respectively.
A frame cell (FC) is defined as a time-frequency interval having a bandwidth corresponding to a predetermined multiple of ΔfTFC of the TFC (e.g., 256 times) and a time duration corresponding to a predetermined multiple of ΔtTFC of the TFC (e.g., eight times). FC can have a maximum bandwidth. The frequency bandwidth and the time duration occupied by FC will be referred to as “FC frequency interval” and “FC time interval”, respectively.
The entire frequency band of the OFDMA-CDM communication system is divided into M FC frequency intervals. First to (M−1)-th FCs are used for the transmission of the packet data, and an M-th FC is used for the transmission of the control information. The number of the FCs used for the transmission of the packet data and the number of FCs used for the transmission of the control information can be variably set depending on the system environment. The number of FCs used for the transmission of the packet data and the number of FCs used for the transmission of the control information are determined by considering that as the number of FCs used for transmission of control information increases, the number of FCs used for transmission of packet data decreases, thereby causing a reduction in data rate. For the convenience of explanation, the FC used for the transmission of the packet data will be defined as a “data FC” and the FC used for the transmission of the control information will be defined as a “control FC.”
In FIG. 2, two different sub-channels A and B are contained in one FC. The “sub-channel” represents a channel over which a predetermined number of TFCs are frequency-hopped before being transmitted according to a predetermined frequency hopping pattern with the passage of time. The number of TFCs constituting the sub channel and the frequency hopping pattern can be variably set depending on the system environment. In FIG. 2, four TFCs constitute one sub-channel.
The OFDMA-CDM scheme will now be described in more detail with reference to FIG. 2.
As described above, the OFDMA-CDM scheme maximizes the performance gain by combining the characteristics of the OFDM scheme and the Code Division Multiple Access (CDMA) scheme. Generally, data corresponding to the sub-carriers assigned to the TFCs are processed using the CDMA scheme and then the resulting signals are processed using the OFDM scheme. The CDMA process includes a process of spreading data by a channelization code uniquely pre-assigned to the sub-carriers and/or a process of scrambling the spread data by a predetermined scrambling code.
In the conventional OFDMA-CDM communication system, data is spread, while pilot signals are not spread. In this case, a process of multiplexing the data and the pilot signals becomes complex. Also, in order to increase the number of base station that can be identified, an interval between the neighboring pilot signals in the frequency domain must vary, from small to large. However, when the data alone is spread, the interval between the pilot signals may be greater than the coherence bandwidth. Generally, the pilot signals are used for the channel estimation as well as the identification of the base station. However, if the interval between the pilot signal is greater than the coherence bandwidth, the performance of the channel estimation is degraded.